MOST WANTED: DRILLING DOWN/MOVIES; The May Factor

By any measure, this summer's box-office receipts are up from the last two years. Depending on how you define ''summer,'' though, ticket sales are up either a little or a lot.

Summer movies used to be defined as ones that opened Memorial Day weekend and later. This summer's box office so far is up only modestly, 10 percent, from last summer's, according to Nielsen EDI.

But ever since Warner Brothers and Universal Pictures found success in 1996 with their joint early-May release of the tornado epic ''Twister,'' studios have tended to release a blockbuster hopeful or two ahead of the Memorial Day clutter -- as Sony did so lucratively last month with ''Spider-Man'' and 20th Century Fox did with ''Star Wars: Episode II.''

By factoring in the early-May megahits, this summer season's ticket sales, through June 16, are up 31 percent from last year's comparable period. TIM RACE